Host associations of the introduced annual root hemiparasite Parentucelliaviscosa in agricultural and bushland settings in Western Australia

Populations of the introduced annual root hemiparasite, Parentucellia visco sa (L.) Caruel (Scrophulariaceae), were examined in habitats where introduc ed, mixed introduced plus native, or solely native species provided potenti al hosts. Presence of haustoria on host roots confirmed parasitism of 17 in troduced and ten native taxa across the sites investigated. Paired plots, o ne with all hosts removed early in the season, the other left intact, showe d overall increases in shoot dry matter of the parasites over 3 months of g rowth. Results indicated a substantial growth benefit to the parasite from continued access to introduced and indigenous plant species, although the p arasite continued growing to a limited extent after removal of hosts. Carbo n isotope discrimination values (delta(13)C) were consistently more negativ e for parasites than for associated hosts, and more negative for parasites in plots containing hosts than in plots from which hosts had been removed. Values for delta(15)N discrimination of shoot dry matter of parasites lay m ostly within the range of those of hosts. The delta(13)C data suggested poo rer water use efficiency of parasite than hosts, while corresponding delta( 15)N data indicated particularly effective uptake of N-containing compounds from the N-2-fixing host, Lotus angustissimus. Where the parasite had acce ss to only one or a few closely related hosts, the composition of the solub le amino fraction of its leaves generally matched that of foliage of host(s ). However, in some instances the parasite accumulated certain amino compou nds to a much greater or lesser relative extent than did its partner host(s ). Where many hosts were being exploited, compositional features of the sol uble amino fraction of the parasite suggested major access to nitrogenous s olutes from specific hosts. Data are discussed in relation to previously pu blished observations on haustorial-derived benefits indicated for other spe cies of root hemiparasites. (C) 2000 Annals of Botany Company.